(1) Field of the Invention
The present invention relates to a photocurrent amplifier circuit and an optical pick-up device in which the photocurrent amplifier circuit is used, and particularly to a technique which realizes a simple and downsized photocurrent amplifier circuit.
(2) Description of the Related Art
These days, optical disc media such as Compact Discs (CDs) and Digital Versatile Discs (DVDs) have been widely used for recording of large-capacity digital information represented by video and audio. As is commonly known, in order to read and/or write information from and/or to these various types of optical disc media (hereinafter simply referred to as media), laser lights with different wavelengths are used depending on the types of such media.
A conventional downsized optical pick-up device compliant with any of CDs and DVDs typically includes a two-wavelength semiconductor laser device which is used as a light source and a single optical system which is used in common for both wavelengths of the laser lights. Subsequently, through the optical system, the respective lasers are projected onto points on the medium, which are different for each of the wavelengths which are dependent on the distance between the emitting points of the lasers having the respective wavelengths. By performing photo-electric conversion of the light reflected from the medium using light receiving devices set for the respective wavelengths, an electric signal is obtained, amplified and outputted.
As a light receiving amplifier device suitable for such an optical pick-up device, a light receiving amplifier device, configured of differential amplifiers set for individual light receiving devices with different wavelengths and an output amplifier which selectively amplifies one of the outputs of the differential amplifiers, is well known (for example, refer to FIG. 3 and FIG. 4 of Patent Reference 1: Japanese Laid-Open Patent No. 2004-22051).
However the circuit characteristics of the conventional light receiving amplifier devices substantially depend on the characteristics of the differential amplifiers in the input stage. Thus, in order to obtain a favorable circuit gain which is different for each of light receiving devices, it is necessary to manage the characteristics of the differential amplifiers for each of the light receiving devices with high precision.
Therefore, in an attempt to streamline the design phase, for example, another circuit configuration can he conceived which gains desirable circuit characteristics by providing the minimal number of amplifier devices required for transmitting a signal in the vicinity of each light receiving device (in other words, an originating point of a signal) as well as by using a gain-switchable output amplifier circuit for each light receiving device in common. However, none of the conventional technology suggests such circuit configuration.
The present invention has been conceived in view of such circumstances, and the object of this invention is to provide a photocurrent amplifier circuit which is suitable for use in such circuit configuration.